RNA polymerase holoenzyme is able to form sigma-dependent rifampicin-resistant pre-initiation complexes at specific sites in Chlamydomonas reinhardi chloroplast DNA (Rochaix et al., 1972); this observation provides a way of looking for factors in C. reinhardi that regulate or modify transcription of chloroplast DNA. Sigma, rho, kappa, CAP and repressor proteins have been found and studied in bacteria systems. Each of these transcriptional regulating proteins would exhibit a distinguishable effect on the in vitro transcription of C. reinhardi chloroplast DNA. By studying in vitro transcription both in the presence and absence of proteins from C. reinhardi, it is possible to detect the activity of any one or all of the above factors. The effect of these factors can be studied in respect to their effect on in vitro transcription of nuclear DNA from the same organism, as well as their effect on procaryotic DNA such as lambda, T4, T7, etc. This would give some insight into the relative similarities or differences in the unique sequences that these factors recognize in DNAs. Factors which affect the transcription of chloroplast DNA can be studied in respect to their site of transcription and translation by applying specific inhibitors of chloroplast transcription (rifampicin) and translation (spectinomycin) to synchronous cultures of C. reinhardi during one vegetative cell cycle.